The present invention relates to a liquid crystal composition for display devices. More specifically it relates to a liquid crystal mixture used in a display device which is operated at high level of multiplexing.
Along with recent rapid progress in the field of information display devices, especially of a portable visual display terminal of a computer, there is an increasing market demand for a thin and compact-type, power-saving display device having as large a display capacity as that of a cathode-ray tube. Satisfying the above-mentioned requirement of thin and compact shape and reduced power consumption, liquid crystal display devices have been widely used in watches electronic calculators, visual display terminals, etc., in which 90 degree-twisted nematic liquid crystal cells have been driven at multiplexing levels up to 100 lines. However, it is considered to be difficult to elevate the level of multiplexing from a duty ratio of about 1/100 in a 90 degree-twisted nematic display mode, moreover, a higher multiplexing level (over 100 lines) theoretically brings an inferior display quality.
Recently there has been proposed a supertwisted birefringence effect (SBE) mode in which a twist angle of a chiral-doped nematic liquid crystal is fixed in a narrow range very close to the boundary where the electrooptical bistability appears between 180.degree. and 270.degree..
It is reported by T. J. Scheffer and J. Nehring that, similar to the conventional 90.degree. TN display cell, the SBE display cell may also be operated in a fast scan addressing mode in which the display responds to the root-mean-square value of the applied voltage. It is also reported that the SBE display shows a better contrast and a wider viewing cone than the conventional TN display. (Refer to Appl. Phys. Lett. 45, 1021 (1984)).
Liquid crystal mixtures for the SBE mode display cell are required to have the following characteristics,
(1) a steepness in transmission versus voltage characteristic; PA0 (2) a high nematic-isotropic phase transition temperature; PA0 (3) an optimum value of an optical anisotropy (hereinafter abbreviated to .DELTA.n) applicable to a cell thickness (abbreviated to d); PA0 (4) a low viscosity; and so forth. PA0 (1) a liquid crystal composition, comprising: PA0 (2) a liquid crystal composition according to item (1), wherein the first component consists of at least one compound expressed by the formula ##STR5## wherein R.sup.1 is an alkyl group of 1 to 8 carbon atoms, and (3) a liquid crystal composition according to item (1), wherein the first component consists of at least one compound expressed by the formula ##STR6## wherein R.sup.1 is an alkyl group of 1 to 8 carbon atoms.
The characteristics described in above items (2) to (4) are also necessary for the conventional TN display cell. The characteristic of item (2) is especially required in the SBE mode for preventing the display from changing hue accompanied by the temperature dependence of the optical anisotropy of liquid crystals; therefore a clearing point of 70.degree. C. or higher is desired for the material used in the SBE mode display. The characteristic of item (4) is effective in reducing a response time of a liquid crystal cell, particularly of a SBE display cell having a twist angle smaller than 240.degree. together with a pre-tilt angle smaller than 6.degree..